Monocytes/macrophages are vital for host-immune responses and have been implicated in the pathogenesis of rheumatoid arthritis (RA). We demonstrated that PI3K/Akt-1-dependent Mcl-1 expression is vital for macrophage survival. Suppression of PI3K/Akt reduced Mcl-1 expression, resulting in apoptosis mediated through the mitochondrial pathway. Forced downregulation of Mcl-1 through antisense oligonucleotides also induced apoptosis, demonstrating that Mcl-1 is essential for macrophage viability. Further, our preliminary data suggested that Mcl-1 may also be regulated by the JAK/STAT pathway in human macrophages. Therefore, we propose to determine the mechanisms by which the PI3K/Akt and JAK/STAT3 pathways contribute to the regulation of Mcl-1 in macrophages. Additionally, we will identify the mechanism by which Mcl-1 protects macrophages by examining the interaction of Mcl-1 with pro-apoptotic molecules, such as Bax in macrophages to delineate the mechanism of mitochondrial dysfunction that occurs following Mcl-1 ablation. Our preliminary data suggests that Mcl-1 may be important in the in maintaining the viability of RA synovial macrophages. Additionally, our preliminary data has revealed that in vitro, Mcl-1 was highly expressed in RA, compared to osteoarthritis (OA), synovial fibroblasts. Mcl-1 was also strongly expressed in the synovium of rats with adjuvant-induced arthritis (AIA). Therefore, we propose to characterize the expression and function of Mcl-1 in the RA joint, examining macrophages and synovial fibroblasts. We propose to determine if the forced downregulation of Mcl-1 will ameliorate experimental arthritis, which would indicate that Mcl-1 is a contributor to the initiation and/or progression of arthritis. Thus, this proposal will delineate the mechanisms regulating the expression and the novel functions of Mcl-1 in macrophages. Further studies are proposed to delineate potential cell type-specific differences between macrophages and normal, osteoarthritis and rheumatoid arthritis synovial fibroblasts. These experiments will provide new and important information concerning the novel role of Mcl-1, which may provide insights that will lead to the development of improved therapy for patients with RA.